The production and release of interleukin 8 (IL-8) from exudative neutrophils may play an important autocrine role in the recruitment of additional neutrophils to inflammatory sites in vivo. Compared with human peripheral blood cells, exudative neutrophils have greatly increased stores of the neutrophil chemoattractant IL-8 but the mechanism for the increase in neutrophil IL-8 during exudation is not defined. This year we have shown that treatment of peripheral blood neutrophils with either the chemotactic peptide fMet-Leu-Phe (fMLF) or leukotriene B4 (LTB4) alone, or fibrinogen alone, results in little increase in the production of IL-8 by peripheral blood neutrophils. However, a chemotactically active dose of fMLF or LTB4 in the presence of a physiological concentration of fibrinogen results in synergistic (200 fold) induction of IL-8 and protein synthesis. The levels of IL-8 obtained are comparable to those observed in exudative cells. Higher concentrations of fMLF are associated with reduced IL-8 mRNA and protein synthesis, without IL-8 degradation, indicating a sensitive regulatory mechanism for IL-8 production. Treatment of neutrophils with fibrinogen and fMLF resulted in eight fold increased steady state levels of mRNA for IL-8 but had little or not effect on the steady state levels of mRNA for macrophage inflammatory protein-1 or monocyte chemoattractant protein-1. These data indicate that fibrinogen can function not only as a substrate in the clotting cascade, but also as an important effector during the evolution of the innate immune response.